Many color-printing devices use cyan (C), magenta (M), yellow (Y), and black (K) colorants, referred to collectively as CMYK colorants. In CMYK output devices, many colors are reproducible by more than one combination of CMYK colorants. When a CMYK device is modeled as an RGB device, each RGB combination must produce a unique CMYK combination. Under-color removal (UCR) and gray component replacement (GCR) techniques are traditionally used to calculate these unique combinations. These techniques are simple to implement, but cannot fully utilize the color gamut possible with the additional K colorant. Other brute-force techniques that search the entire CMYK signal space for desirable combinations can produce good results but are unsuitable for real-time implementation.
A number of advantages are gained by using a K colorant in addition to CMY colorants. These include denser blacks, better shadow details, and easier control of neutral color balance. In profiling a CMYK printer for color management, combinations of the C, M, Y, and K colorants are computed to reproduce colors specified in a colorimetric space, such as CIELAB or sRGB. As previously noted, some colors may be reproducible via multiple CMYK combinations. In UCR and GCR techniques, RGB signals are first inverted to obtain nominal CMY signals. Then, based on the composition of the CMY colorants, the K colorant is used to replace certain amounts of the CMY colorants. The amounts of K colorant used and CMY colorants removed usually depend on the minimum of the input C, M, and Y amounts. A typical implementation uses one black generation (BG) curve and three UCR curves. These curves may be implemented as 1-D LUTs (look-up tables) where the minimum of the C, M, and Y signals is used as the index into the LUTs. This provides a model for converting nominal RGB signals into printer CMYK signals.
With such a model, the printer may be considered to be an RGB printer, where the RGB signals have a one-to-one correspondence with the colors specified in a colorimetric space for all the in-gamut colors. To profile an RGB printer, an RGB printer target containing color patches, which extensively sample the input RGB cube, may be generated, and the output CIELAB values then measured. Based on the measurement data, a printer profile may be created which converts colors specified in a colorimetric space to the printer RGB space. Gamut mapping techniques are used to reproduce out-of-gamut colors. Clearly, an RGB printer model is very useful in the modular design of a printer profiling process. Such a technique hides the complexity of the RGB-to-CMYK conversion process and provides users a more intuitive RGB input color space. There are also applications, such as Windows® Graphical Display Interface (GDI) printing, where the printer is only addressable as an RGB device, which necessitates the construction of RGB printer models.
The simple UCR/GCR technique mentioned above does not provide enough flexibility to select a “best” CMYK combination to reproduce a specified color. The color gamut of the resulting RGB printer is also reduced, due to the inability of the simple technique to use certain combinations of CMYK values.
More sophisticated methods for controlling the CMYK usage generally search the entire gamut of the CMYK printer to obtain the desired CMYK signals. Consider a color specified in a colorimetric space: if it is an in-gamut color, the search will find one or more CMYK signal vectors that reproduce the input color. These CMYK signals may be sorted by the amount of K colorant used, from the minimal to the maximum amount. The desired CMYK signals are obtained by specifying the relative amount of K colorant to be used. One way to do this is to use a coefficient to specify the amount of K to be used relative to the minimum and maximum K amounts in the CMYK signal vectors found. This coefficient may be designed to be a function of position in the input color space to achieve optimal output image quality. These methods take advantage of the full CMYK printer gamut.
However, constructing a printer target that extensively samples the entire printer CMYK space, measures the printed target in a colorimetric space, such as CIELAB, and interpolates to find all possible CMYK signals for each in-gamut color specified in the colorimetric space is a computationally intensive, “brute force” method. Furthermore, the output CMYK signals are not associated with input colors in an RGB signal space, but with colors in colorimetric space. Therefore, these methods alone are not sufficient for modeling a CMYK printer as an RGB printer, although they are useful in creating a 3-D color LUT that converts colors in a colorimetric space directly to printer CMYK signals. Of course, gamut mapping techniques must also be incorporated into this LUT creation process.
The use of a black (K) colorant in printing achieves a higher density than that achievable by combining CMY colorants alone. The problem to be solved is how to determine the amount of black colorant to be used based on an RGB or CMY input, and how to adjust the other colorants once black colorant is introduced. As previously noted, the prior art offers two main ways to solve this problem: (1) UCR-type techniques, and (2) search techniques. The related patent application describes a method of using interpolation and multiple black generation LUTs on the boundaries and the center diagonal of a CMY cube in a method for converting input RGB signals into CMYK signals.
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